1. Field of the Invention
This invention relates to tunable single-mode lasers and more specifically to a coupled-cavity tunable glass laser using a long-long coupled cavity scheme with a coupling waveguide.
2. Description of the Related Art
Rare-earth doped optical waveguides such as fibers or planar waveguides are used in amplifiers and lasers for telecommunications because they provide high optical gain over a broad spectral range. In the simplest laser geometry, a gain medium is placed in a cavity defined by two reflectors. The cavity has periodically spaced longitudinal modes with a wavelength spacing xcex94xcex given by:                               Δ          ⁢                      xe2x80x83                    ⁢          λ                =                              λ            2                                2            ⁢            nd                                              (        1        )            
where n the refractive index of the gain medium and d the length of the cavity. The laser only oscillates at a frequency (or frequencies) that coincides with one (or several) of these cavity modes. Which one and how many modes reach threshold depends on the details of the gain medium. In the ideal case of a purely homogeneously broadened system only the mode that is closest to the gain maximum will oscillate and saturate the optical gain, i.e. pin the gain to the value that is necessary to reach the lasing threshold for this one mode. Even though rare earth doped glasses at elevated temperature are often considered to be dominantly homogenously broadened, inhomogeneous broadening is an important factor in these materials and for closely spaced longitudinal cavity modes, many lasing modes will oscillate.
To achieve single-mode operation, the active cavity length can be reduced so that the mode spacing exceeds the gain bandwidth. Since this approach limits the cavity length to a few hundred micrometers, the output power of such a laser is very small and typically tens of microwatts. [K. Hsu, C. M. Miller, J. T. Kringlebotn, E. M. In Taylor, J. Townsend, and D. N. Payne, xe2x80x9cSingle-mode tunable erbium:ytterbium fiber Fabry-Perot microlaserxe2x80x9d, Optics Letters, 19, 886 (1994), K. Hsu, C. M. Miller, J. T. Kringlebotn, and D. N. Payne, xe2x80x9cContinuous and discrete wavelength tuning in Er:Yb fiber Fabry-Perot lasersxe2x80x9d Optics Letters 20, 377 (1995)] With the development of waveguide/fiber Bragg gratings, at least one of the broad band reflectors can be replaced with a compact wavelength selective Bragg grating, which provides feedback over a spectral width that is much narrower than that of the gain medium. With a typical spectral bandwidth of these reflectors of about 0.1-0.2 nm, active cavities as long as a few centimeters with output power of several tens of milliwatts have been demonstrated [W. H. Loh, B. N. Samson, L. Dong, G. J. Cowle, and K. Hsu, xe2x80x9cHigh Performance Single Frequency Fiber Grating-Based Erbium:Ytterbium-Codoped Fiber Lasersxe2x80x9d, Journal of Lightwave Technology, Vol. 16, No. 1, p. 114 (1998), D. L. Veasey, D. S. Funk, N. A. Sanford, and J. S. Hayden, xe2x80x9cArrays of distributed-Bragg-reflector waveguide lasers at 1536 nm in Yb/Er codoped phosphate glassxe2x80x9d, Applied Physics Letters, Vol. 74, No. 6, p. 789 (1999)]. Longer cavities could provide even higher power output but also lead to a large number of longitudinal cavity modes inside the selected wavelength band and therefore to multimode operation of the laser.
Limited wavelength tunability can be achieved by controlling the temperature or length of the fiber Bragg grating to shift the spectral position of the reflection peak. Owing to the very small temperature dependence of the glass, the thermal tuning range of these lasers is on the order of a few nanometers only. Strain or compression tuning of specially designed fiber Bragg grating can result in larger tuning ranges. However, this kind of tuning is typically done using piezoelectric transducers and the effects of creep and hysteresis limit the wavelength reproducibility and so far have prevented the practical implementation of these lasers. Distributed feedback lasers also demonstrated single frequency operation with a high degree of side mode suppression. Due to the fixed grating, wavelength tunability of these lasers is, however, problematic as well.
Coupled cavity lasers, in which an external cavity (active or passive) is coupled to an active laser cavity, have the potential of combining mode selectivity with the possibility of wavelength tuning. The external cavity acts as a periodic wavelength dependent mirror providing minimum cavity loss only for certain longitudinal modes of the active laser cavity. The performance of such lasers depends on the relative optical length of the cavities where the distinction can be made between long-long and long-short cavity assemblies. In the case of a long-short coupled cavity, one cavity is short enough so that its mode spacing is large compared to the spectral width of the gain profile. All but the mode that is inside the gain profile of the active medium are suppressed. To tune the wavelength of such a laser over a region comparable to the width of the gain spectrum, the optical length of the short cavity has to be changed considerably. An example of such a short external cavity fiber laser is given in U.S. Pat. Nos. 6,137,812 and 5,425,039. By placing a fiber assembly into fiber ferrule alignment fixtures, the length of a short air gap can be changed by piezoelectric means. Long-long cavities, on the other hand, have the advantage that only small changes in the optical path of either cavity are needed to obtain a broad tuning range. This effect is called the Vernier effect. In addition to that, long-long cavities are able to provide larger output powers. The major drawback of known long-long devices has been mode stability.
The primary application of long-long coupled-cavity structures is found in fixed wavelength semiconductor lasers to achieve large side-mode suppression (see U.S. Pat. Nos. 4,608,697 and 4,622,671). As shown in FIG. 1 of the ""697 patent, active sections 1 and 2 of slightly different length are separated by an air gap 3 created by etching or cleaving a monolithic heterostructure. Mirror facets are formed on the ends of each active section. Coupling of the two cavities is achieved through free space. If the gap between the two cavities is large, high diffraction losses are introduced and coupling between the two cavities is weak. Therefore, in this geometry the length d of the gap is typically chosen to be less than ten times the wavelength 10 xcex. Coupling between the two cavities depends also dramatically on the distance between them and is maximum when:                     d        =                  M          ⁢                      xe2x80x83                    ⁢                      λ            2                                              (        2        )            
where d is the gap distance and M is an integer. In this case the optical fields of the two cavities interact constructively and the situation corresponds to a transmission maximum of the coupling Fabry-Perot cavity 3.
The combined cavity modes are often called supermodes and the mode spacing xcex9 of the combined long-long coupled cavity supermodes is given by:                     Λ        =                              λ            0            2                                2            ⁢            Δ            ⁢                          xe2x80x83                        ⁢            nd                                              (        3        )            
where (xcex94nd) is the difference in the optical path length between the two cavities and xcex0 the wavelength.
Due to limitations in achieving a homogeneous pump (current) profile, telecommunication semiconductor lasers usually have active cavities of only a few hundred micrometers in length. Typically the difference in cavity length is less than 50 micrometers and the mode spacing of the supermodes is on the order of 10 nm when working, for instance, inside the telecommunications C-band, around 1550 nm. This creates a situation in which only a few supermodes exist inside the gain spectrum and all but one of the supermodes are suppressed by gain roll-off. A short air gap of about 1.5 xcexcm has a free spectral range (FSR) of more than 250 nm, which also corresponds to the separation of peaks in the coupling coefficient. Only one broad transmission or coupling peak exists inside the spectral width of the gain profile. The coupling cavity is effective to create the supermode structure but has no role in selecting a particular supermode.
Limited tunability of semiconductor coupled-cavity lasers is achieved by tuning one or both of the active cavities via pump current to move the supermode back-and-forth. Useful tuning ranges of about 10 nm have been demonstrated (W. T. Tsang, N. A. Olsson, R. A. Linke, and R. A. Logan, xe2x80x9cStable single-longitudinal-mode operation under high-speed direction modulation in cleaved coupled-cavity GaInP semiconductor lasersxe2x80x9d, Electronics Letters 19p. 415 (1983)). Tunable semiconductor lasers are also very low power, less than 5 mW, due to the short length of the active cavities.
Despite intensive modelling of coupled-cavity lasers, practical implementation of single mode operation of such lasers has proven to be difficult. (see for instance Henry et al. (1984) [C. H. Henry and R. F. Kazarinov, IEEE J. Quantum Electron. QE-20, 733 (1984)], Coldren et al. (1983) [L. A. Coldren, K. J Ebeling, B. I. Miller, and J. A. Rentschler, xe2x80x9cSingle Longitudinal Mode Operation of Two-Section GaInAsP/InP Lasers Under Pulsed Excitationxe2x80x9d IEEE J. Quantum Electron. QE-19, 1057 (1983)], Marcuse et al. (1984) [D. Marcuse and T. P. Lee, xe2x80x9cRate Equation Model of a Coupled-Cavity Laserxe2x80x9d IEEE J. Quantum Electron. QE-20, 166 (1984)], Streifer et al. (1984) [W. Streifer, D. Yevick, T. L. Paoli, and R. D. Burnham, xe2x80x9cAn Analysis of Cleaved Coupled-Cavity Lasersxe2x80x9d, IEEE J. Quantum Electron. QE-20, 754 (1984)and xe2x80x9cAnalysis of Cleaved Coupled-Cavity (C3) Diode Lasersxe2x80x94Part II: Frequency Modulation, Above Threshold Operation, and Residual Amplitude Modulationxe2x80x9d, IEEE J. Quantum Electron. QE-21, 539 (1985)], Coldren et al. (1984) [L. A. Coldren, T. L. Koch, xe2x80x9cAnalysis and Design of Coupled-Cavity Lasersxe2x80x94Part I: Threshold Gain Analysis and Design Guidelinesxe2x80x9d IEEE J. Quantum Electron. QE-20, 659] (1984) and xe2x80x9cAnalysis and Design of Coupled-Cavityxe2x80x9d) This is enforced in U.S. Pat. No. 4,896,325 that states xe2x80x9cSome time ago, it was felt in the art that any wavelength over the gain bandwidth of a laser could be selected by a properly designed two-section, coupled-cavity structure using the combined mode-jump/continuous tuning philosophy. We know now that it is not possible to get sufficient spurious-mode suppression and unambiguously select a particular wavelength over the entire band.xe2x80x9d The main difficulty in achieving the stability necessary for these lasers stems from the refractive index variations associated with the injection of current and temperature changes causing frequency instability. It is well known that in semiconductor lasers only small changes in the temperature or the pump power induce large refractive index changes.
In view of the above problems, the present invention provides a high power tunable single-mode laser.
This is accomplished with long-long coupled-cavity glass laser in which the two active waveguide cavities are coherently coupled using a passive waveguide cavity. Pump light is coupled into the active cavities to invert the gain medium and provide the necessary amplification to sustain lasing. Typically, one of the active cavities will be optically pumped slightly above its lasing threshold while the other one is excited to transparency close to its single cavity threshold. In another mode of operation, both active cavities are pumped above their individual threshold. In the case of strong coherent coupling between the two active cavities, new lasing eigenmodes of the coupled system are created. The active and passive waveguide cavities are of sufficient length so that multiple supermodes and multiple peaks in the coupling coefficient are created over the width of the gain spectra. The supermodes are gain flattened so that the one supermode that coincides with a peak in the coupling coefficient will oscillate. In other words, a first Vernier effect creates the supermode structure and a second Vernier effect selects a particular supermode.
Tunability is achieved by changing the optical path length of either the passive or active waveguide cavities. A broad tuning range is achievable because only small variations in path length are required to match a different supermode to a different peak in the coupling coefficient. Stable single-mode operation is made possible by the relative insensitivity of glass to fluctuations in temperature and pump power as compared to semiconductor lasers. High power levels are attained because the active waveguide cavities can be relatively long without mode hops and are preferably formed of a high gain Er:Yb co-doped phosphate glass.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings in which: